1. Field of the Invention
This invention relates generally to inorganic metal chalcogen cluster precursors and methods for forming colloidal metal chalcogenide nanoparticles using the inorganic metal chalcogen cluster precursors.
2. Description of Background
Metal chalcogenides are a range of compounds that contain a metal and a Group VI element such as sulfur, selenium, or tellurium. These compounds may be in binary, ternary, or quaternary form. Colloidal nanoparticles formed of the metal chalcogenides have been used to fabricate a wide range of optical and electronic devices including light emitting diodes, solar cells, optical recording media (phase change), thin film transistors, and the like. These materials have also been used as luminescent “tags” for biological labeling and have exhibited lasing and other nonlinear optical effects. Colloidal metal chalcogenide nanoparticles have been synthesized by mixing, in solution, precursors containing the metal element(s) with a precursor containing the chalcogen element and at least one surfactant that provides for dispersion in the solution. A controlled precipitation reaction occurs, wherein the various different precursors react to form the metal chalcogenide nanoparticles and the surfactant coats the particle surface to limit growth. The precipitation reaction normally requires high temperatures (e.g., temperatures of 250 to 360° C.) in order to decompose the precursors, overcome the energetic barrier to nucleation, and form nanoparticles, particularly if crystalline nanoparticles are desired.
Some of the known problems with the co-precipitation route to nanoparticle synthesis include, but are not limited to, the high temperatures noted above that are needed to initiate the reaction between the decomposing precursors (i.e., the reaction between the metal precursor and the chalcogen precursor) and that suitable precursor combinations must be discovered that are both soluble in the reaction mixture and that decompose at a sufficiently low temperature, i.e., at temperatures below at or below the boiling point of the solvent. While a few chalcogen precursors have been found suitable for a wide range of metal chalcogenides, a suitable precursor must be discovered for each new metal to be used. This has led to extensive development of nanoparticle synthesis for certain metals such as cadmium (Cd), zinc (Zn), or mercury (Hg), with limited success in using analogous synthetic methods for other metal chalcogenides. In some cases, it has proven difficult to identify conditions and precursors for the preparation of certain desirable metal chalcogenide nanoparticles using the co-precipitation process described above.
Another process for forming colloidal metal chalcogenide nanoparticles includes the use of a so-called single-source precursor. Single-source precursors are generally molecular precursors with the desired bonds already formed prior to the nanoparticle synthesis. This type of precursor can allow for the formation of nanoparticles with compositions that may be difficult to make and/or control through the traditional co-precipitation route, which involves bond making and breaking. Single-source precursors can also have the advantage of being more stable than their reagent counterparts. An air-stable precursor for the synthesis of cadmium and zinc sulfide/selenide nanoparticles has been reported, whereas the prior syntheses of these nanoparticles involved air-sensitive compounds. Another benefit of single-source precursors is that they can allow for synthesis of nanoparticles from relatively harmless reagents.
One disadvantage with single source precursors is that the synthesis of currently known precursors often involves highly reactive/toxic reagents and can be quite complex. As a result, the precursors are compositionally specific to the desired nanoparticles. A good example is the use of single-source precursors to synthesize CdSe nanoparticles. For example, some researchers have used diselenocarbamato cadmium complexes whereas others have used metal-chalcogenide thiophenolate clusters to synthesize the particles. Even more recently, the use of a thiocarbamoyl hydrazine cadmium complex for CdSe nanoparticle synthesis has been reported. In all of these syntheses, the precursor used can only make one type of nanoparticle. If nanoparticles with different characteristics, surfactants or composition are desired, the precursor must be completely redesigned and remade.
Accordingly, there is a need for a different class of single-source precursors suitable for the synthesis of colloidal metal chalcogenide nanoparticles in order to overcome the limitations of existing single-source precursor and expand the possibilities for synthesis of metal chalcogenide nanoparticles.